When aphids attack, plants don't just stand there and take it: New research has found they send out a chemical scream for help.

And when that help arrives in the form of ladybugs or other aphid predators, the aphids ring their own chemical alarm bell and scatter.

Now researchers have manipulated this system and altered a plant so that it rings the aphid alarm bell all the time - like the boy who cried wolf. The result is aphids eventually ignore the alarm and don't scatter, making them very easy pickings for ladybugs.

"It's an arms race between plants and aphids," says Dr Martin De Vos, formerly of the Boyce Thompson Institute in New York, and now at the firm Keygene in the Netherlands.

Most aphid species run for it when they detect a pheromone called (E)-Β-farnesene (EBF), which is released by aphids when they are attacked. Some aphid predators home in on that signal as well, sort of like how blood in the water attracts more sharks.

"It's always been on (plant) breeders' minds that maybe they could use this for aphid resistance," says De Vos.

This is useful not because aphids themselves are particularly damaging, but because they transmit dangerous plant viruses, he says.

"By raising aphids on plants that were genetically engineered to produce the aphid alarm pheromone, we created 'fearless' aphids that no longer run away," says Dr Georg Jander, a co-author on the paper. "It's a habituation phenomenon similar to being left in a room with a skunk; after a while you don't smell the skunk anymore."

What's more, the habituated aphids actually grow faster than ordinary aphids, likely because they spend less time responding to false alarms, says Jander. When ladybugs arrive, however, the fearless aphids get eaten more easily because they don't know that they should run away.

Fearlessness

The researchers also showed that exposure to the alarm pheromone causes gene expression changes in the aphids. In just three generations (not long for aphids, which are born pregnant), aphids were habituated to EBF. Likewise, they lost their "fearlessness" in three generations if raised on non-EBF-producing plants.

"It's really very exciting stuff," says Professor John Pickett of Rothamsted Research in the UK. Earlier work had tried exposing aphids to short bursts of EBF, but hadn't gone so far as to discover the effects of habituating the aphids to the alarm. "The fact that there is increased predation of the habituated aphids - that's really exciting."

"Some other plant species, including potato, can produce EBF, and the levels reported vary among (varieties)," says Professor Sanford Eigenbrode, chair of the entomology division at the University of Idaho.

It may even be possible that this could explain why some kinds of potatoes are more susceptible to aphids and others less so.